The present invention generally relates to fiber optic gyroscopes. More particularly, the present invention relates to a sensing coil and hub assembly of a fiber optic gyroscope.
Fiber optic gyroscopes use optical fibers for transmitting light waves in order to determine rotational movement. Optical fibers are strands of glass fiber which are used to transmit optical signals over long distances with low loss and distortion. Optical fibers are typically comprised of an inner glass core, an outer glass cladding, where the inner glass and the outer glass have different optical indices of refraction, and a plastic coating, or jacket, placed over the glass. Optical fibers may be tightly and specifically wound into a cylindrical structure known as a sensing coil.
In a fiber optic gyroscope, light traveling through these fibers is divided by a beam splitter into two counterpropagating waves, propagating in phase with respect to each other, which enter different ends of a sensing coil. When the fiber optic gyroscope is not rotating, the two waves return exactly in phase after having traveled the same path in opposite directions. When the fiber optic gyroscope is rotating, or more specifically, when the coil is rotating, one wave travels a longer path in the coil than the other wave to get from one end of the coil to the other, thus offsetting the phases of the two waves with respect to each other. The speed of the rotation of the coil does not affect the time the light takes to travel between any two points along the coil because the speed of light is independent of the velocity of the medium through which the light travels. Hence, the difference between the phases of the two counterpropagating waves indicates the rotation of the coil and is measured via electrical circuitry.
Environmental factors may adversely affect the phase difference between the counterpropagating waves. That is, environmental factors such as temperature and mechanical strain, may create a bias between the phases of the counterpropagating waves such that the output of the sensing coil may yield a phase difference between the two waves which is indistinguishable from a rotation-induced phase difference. Another environmental factor, vibration, may induce motion of the sensing coil with respect to the hub, resulting in spurious output. Environmental factors that are constant can be compensated for in determining the actual rotation of the sensing coil. In other words, at zero input, the output of the sensing coil may yield a non-zero output as a result of environmental factors which appears as a phase difference between the two counterpropagating waves. If the non-zero output is constant, it may be adjusted for such that an accurate phase difference indicating only coil rotation may be determined. However, a non-constant non-zero output may not be adequately adjusted for and may cause the sensing coil to produce a result yielding a phase difference based both on rotation and on environmental influences.
As stated above, one such environmental factor is temperature. A fiber optic gyroscope is exposed to various temperatures during its operation. Temperature variations affect the sensing coil in two ways: first, the sensing coil undergoes mechanical strain as a result of differential thermal expansion; second, the optical transmission properties of the optical fiber change with temperature.
A sensing coil of a fiber optic gyroscope is incorporated into the structure of the gyroscope via a coil-supporting structure known as a hub. The hub and coil are both substantially cylindrical structures oriented about a center axis where the hub has a smaller radius relative to the radius of the coil. Typically, hub material and sensing coil material exhibit different coefficients of thermal expansion. For example, the thermal expansion of a hub made from an isotropic material may occur relatively uniformly in both the axial direction with respect to the center axis and in the radial direction. However, the thermal expansion of a coil may occur non-uniformly in the axial direction and in the radial direction. More specifically, a coil may exhibit a relatively large thermal expansion in the axial direction in a manner similar to that of the isotropic hub; however, the coil may exhibit a relatively small or negative thermal expansion with respect to the isotropic hub in the radial direction. As a result, in this example, when a fiber optic gyroscope is exposed to a temperature change such that expansion of its coil and hub occur, because the hub is radially expanding faster than the coil, strain is imparted to the sensing coil, and in the extreme, may create folds, cracks or other mechanical instabilities in the coil.
Thus, in selecting a hub for attachment to a coil it is advantageous if the thermal expansion coefficient of the coil and the hub are approximately equal. It is relatively simple to closely match the thermal expansion coefficients in only the radial direction or only the axial direction, but it is difficult to find a suitable hub material which closely approximates the thermal expansion coefficient of the coil in both directions while also being suitable for connecting the coil to the structure of the gyroscope.
In addition to the selection of materials in a coil/hub adhesive system, the manner of adhesion is also problematic. Continuously bonding the hub to the coil may impose undesirable strains on the coil over temperature variations because the adhesive acts as a constrained fluid, applying hydrostatic pressure on the coil.
In addition to temperature, vibration is an environmental factor which also affects the output of the gyroscope. Vibration is induced into the coil from its attachment to the hub. This vibration contributes to bias between the phases of the counterpropagating waves which are output from the sensing coil. Thus, there is a need for reducing the vibration experienced by the coil in order to obtain more accurate coil rotation information.
Additionally, there is a need to develop a realistic, manageable adhesive system which will work within a range of tolerances for parts. More specifically, machined parts such as hubs and coils typically vary to some degree in size, shape or the like. Typically the parts are manufactured to have component tolerances which are within determined acceptable tolerance levels. It would be cost prohibitive to customize a part for its specific use, i.e., to custom-machine each hub based on the final size and shape of each coil. Therefore, there is a need to design an adhesive system which is self-adaptive to variations in part sizes which are within acceptable tolerance levels for affixing a coil to a hub.
In sum, several factors may be considered in optimizing a coil/hub adhesive system. Typically these factors are the environmental factors discussed above, namely thermal expansion and contraction, vibration and mechanical strain. However, many other factors also affect the specific selections in a particular coil/hub adhesive system. These include, among others, the particular application in which the fiber optic gyroscope will be operated, the tolerances associated with the various manufacturing processes and the costs incurred with the selection of materials and processes. Typically, the most efficient coil/hub adhesive system results from balancing these factors because not all factors may be optimized for a particular application. For example, hub materials may be selected which approximate the thermal expansion characteristics of the coil in an axial direction or a radial direction, but it is difficult to create a material that approximates the thermal expansion characteristics of the coil in both directions. Alternatively, composite materials may be designed to closely approximate the thermal expansion characteristics of the coil; however, such composite materials introduce complications in the manufacturing of finishing operations.
Accordingly, it is an advantage of the present invention to improve the performance of a fiber optic gyroscope by minimizing environmental influences, such as temperature, vibration and mechanical strain, on a sensing coil in order to obtain more accurate rotational information about the coil.
Another advantage of the present invention is to design an adhesive system for affixing a sensing coil to a hub which is less sensitive to variations in part sizes which are within acceptable tolerance levels.
Another advantage of the present system is to design an adhesive system which minimizes the build up of hydrostatic pressure on the coil by allowing the adhesive to expand into grooves located between the bonding areas between the coil and the hub.
The above and other advantages are carried out in one form by an adhesive system for use in a fiber optic gyroscope comprising a free-standing substantially cylindrical sensing coil having an inner coil surface, a rigid substantially cylindrical coil-supporting structure or hub having a cylindrical wall and an outer surface, a plurality of grooves formed into the hub""s outer surface, a plurality of slots formed into the hub""s outer surface and an adhesive affixing the hub to the coil. A plurality of manifolds may be included in the hub""s cylindrical wall for introducing the adhesive into the slots of the coil/hub assembly. More specifically, the adhesive may be injected into the manifolds, travel through the slots into a gap existing between the coil and the hub. Excess adhesive may flow into the grooves to separate bonding areas between the coil and hub. Different orientations and sizes of the grooves and slots, different types and amounts of adhesive, and different thicknesses of adhesive between the coil and hub may be used to achieve suitable adhesion.
In an alternate exemplary embodiment, the adhesive system may further include a stress buffer and additional adhesives to further minimize environmental influences on the coil. More specifically, a first adhesive affixes the inner surface of the buffer to the outer surface of the hub. A second adhesive affixes the outer surface of the buffer to the inner surface of the coil. The first adhesive is preferably flexible or compliant and the second adhesive is preferably rigid to minimize coil vibration.